This chase aircraft is tracking 100% SAF’s emissions performance

The latest test flight of an A350 operating on 100% sustainable aviation fuel (SAF) has mobilised DLR’s Falcon 20E to capture and analyse in-flight data. Flying within 100 metres during the test campaign, the two aircraft are already delivering invaluable insight, the preliminary observations of which point to the positive impact of SAF on aircraft emissions. 

On this pleasant spring day, the sky is a welcoming blue as an A350 glides through the fluffy white clouds above the South of France. 

Then, suddenly, a smaller aircraft ascends rapidly into view, taking up position just behind the A350. As the larger, commercial jet twists and turns across the sky, its smaller companion mirrors its every move, getting as close as 100 metres away – an intimate airborne dance that is rarely seen.

The aerial two-step continues for about three hours, as the two aircraft weave in and out of the clouds above Nice, Marseille and Corsica. Then, just as suddenly as they joined up in the air, the two aircraft part ways and fly off in different directions. 

Soon, the A350 touches down in Toulouse, France at Blagnac airport. A bright orange logo, emblazoned with the words “Flight Lab,” is clearly visible on its tail. This particular aircraft is, in fact, an Airbus test aircraft. And more importantly, it has just completed a noteworthy test flight as part of the Emission and Climate Impact of Alternative Fuels (ECLIF3) project led by Airbus, together with Rolls-Royce, DLR, Neste, the UK’s University of Manchester and the National Research Council of Canada. 

Indeed, this A350 has the distinction of having operated both of its jet engines on 100% SAF. This time, it is the emissions coming out of the aircraft that are of most interest. And this is precisely where the smaller, ‘chase’ aircraft comes in.

We can test the engines and the fuel system on the ground, but to gather the full emissions data necessary for this programme to be successful, there's only one way to do that and it’s to fly a real aircraft in real conditions.

Mark Lewis, Airbus Test Pilot

Capturing in-flight emissions data: the chase is on

A chase aircraft is an aircraft that follows an airborne subject to make real-time observations, capture images during flight or collect engineering data. This type of in-flight testing has been in use since the 1960s, a technique pioneered by NASA and the United States Air Force. 

For the ECLIF3 project, DLR’s modified Dassault Falcon 20E-5 plays the starring role as the chaser aircraft. The Falcon can carry a total of 1,100 kg of scientific instruments inside and outside the cabin, as well as under the wings. It is robust and versatile, capable of flying close to thunderstorms or just 30 metres behind powerful jets like the A350. First introduced in 1976, the Falcon has become a crucial vehicle through which to better understand the effects of aircraft emissions on our atmosphere. 

Now, the DLR Falcon’s mission is to measure emissions from an A350 test aircraft powered by Rolls-Royce Trent XWB engines fuelled with 100% SAF. Unlike today’s SAF blends, this unblended type is not mixed with any fossil fuels. Analysing its behaviour and performance in real-world scenarios is crucial to achieving future certification. And the Falcon is equipped to do just that: its multiple probes and sensors can capture emissions data and feed it into on-board scientific instrumentation for analysis.  

“We can test the engines and the fuel system on the ground,” says Mark Lewis, Airbus Test Pilot. “But to gather the emissions data necessary for this programme to be successful, there's only one way to do that and it’s to fly a real aircraft in real conditions.”

Early results show promise

In-flight testing of the A350 offers the advantage of characterising direct and indirect engine emissions. These emissions include particulates, as well as contrails – clouds of ice crystals that can form behind an aircraft at high altitude. 

According to Toby Wells, Airbus’ Head of Future Fuels, preliminary observations from the test flight with the chase aircraft suggest unblended SAF can have a positive impact on aircraft emissions.  

“As we predicted, the particulate emissions of the aircraft were lower when using the 100% SAF,” he says. “These particulates play a role in the formation of contrails, which contribute to aviation’s climate impact, so we’re pleased to have a solution to address those emissions.”  

Toby underscores there is still a lot more data to analyse to gain a full understanding of unblended SAF’s emissions performance. However, initial results are indeed very promising.  

Findings from the study are first expected to be published towards the end of next year